The present invention relates to a valve with ball of controlled deformation.
Ball valves are fluid shut-off devices which are widely used in all types of processes and all branches of engineering. These ball valves are used, for example, in water, oil, gas transport systems, in hydrocarbon production plants, in chemical, petrochemical plants and in electrical energy generating equipment.
It should also be noted that ball valves are available and may be used in a wide range of sizes, from the smallest to those which exceed 60 for pressures up to hundreds of bars.
In such shut-off devices, i.e. ball valves, the fluid seal is effected between the ball and the two seats, each of which is equipped with a circular insert of deformable material which is clamped between the two metal surfaces.
The function of the inserts is to compensate, by their own deformations, for the elastic deformations, the small geometrical errors and the roughnesses which are always present on the surface of the ball.
In general, the force required to flatten the insert, and hence to produce the seal, is supplied, as shown in FIG. 1 relating to a ball valve of the prior art, to a modest extent by a system of springs (Fm) and to the greatest extent by the pressure exerted by the said fluid on the seat (Fs).
The inserts are normally made of elastomeric (nitrite, viton etc.) or thermoplastic (PTFE, nylon etc.) material which can ensure optimum performance (i.e. high levels of sealing) as long as the operating conditions do not become particularly onerous.
Examples of such onerous and limit conditions are those which may be caused by the presence of particularly xe2x80x9cdirtyxe2x80x9d fluids which entrain abrasive particulates. A further example of onerous conditions is one in which the valve is required to operate for a prolonged period in the partially open condition, with the fluid which, flowing at high velocity, violently laps the insert or conditions in which high temperatures are present.
In these cases in which the use of elastomeric or thermoplastic inserts becomes critical and an adequate durability of the system cannot be guaranteed over time, use is made of completely metallic seal elements.
The metal surfaces of the seats and of the ball, which are to provide the seal and on which the reciprocal sliding takes place, are hardened for this purpose, by heat-treating the base materials. Alternatively these surfaces may be provided with hard superficial facings obtained by welding or other more advanced technologies (plasma, HVOF etc.).
In this type of valve, in which the contribution towards obtaining a good seal which is made by the elastic deformation of the insert is lacking, it is necessary to have recourse to extremely accurate machining of the surfaces of the seats and of the ball, which come into contact with each other, so as to minimize any geometrical errors and surface roughnesses which are present.
Again from a reference to FIG. 1, it is possible to see that when the ball valve is closed and subject to pressure (p), the ball deforms in a non-uniform manner because its geometry is not axially symmetrical. In fact the force lines due to its state of stress pass through the ball from the cap (a) on which the pressure acts, in correspondence with the supports of the two hubs (b), and the points of the ball belonging to the contact circumference with the seat (d) shift in non-uniform manner from (d) to the curve (e), while the external surface of the ball assumes the shape (g) and the passage hole of the fluid the shape (f), both denoted by dotted lines in FIG. 1.
An observation of the behavior of the valve in this condition subject to pressure enables one to note that the ball is much more rigid in the two zones, top and bottom, closest to the hubs (b) with respect to the central zone which is rendered more elastic by the passage hole of the fluid of diameter 0.0.
This deformation of the ball is obviously greater, the higher the working pressure (p).
But the more the deformation of the contact circumference from (d) to (e) differs from point to point, the more problematical the possibility of keeping the metallic contact between ring-seat and ball, or rather a seat/ball contact pressure, as uniform as possible so as to guarantee good sealing conditions over the 360xc2x0.
Attempts are normally made to obviate this phenomenon by stiffening the ball, i.e. with the same passage dimension (diameter "PHgr"0), increasing the external diameter thereof (diameter "PHgr"1) All this does, however, involve greater size, weight and cost of the valve with very modest benefits as regards its structure and operation.
Further attempts have been made to improve the operation of ball valves in various ways. An attempt is made to limit the drawback by, for example, making the seats more deformable, in a manner compatible with their state of stress, i.e. capable of Following the deformations of the ball in improved manner under the thrust of the springs (Fm) and the pressure (Fs), as shown in FIG. 2, an embodiment of the prior art.
In this case also, however, calculations and parallel experiments show that these interventions make modest contributions towards a genuine resolution of the problem.
The object of the invention is therefore to provide a ball valve in which it is possible to obtain a very tight seal even when the ball valve is subjected to particularly high pressure.
A further object of the invention is to provide a ball valve which, while remaining very simple in structure and of modest cost, fully meets the operating and sealing performances which are necessary and required.
These and further objects according to the invention are achieved by producing a valve with ball of controlled deformation for fluid shut-off comprising an external containment body, inside which is arranged a shutter element, movable between a closure position and an opening position of the valve, the shutter element cooperating with one or more seats and providing for seal elements with respect to the containment body and to the shutter element, characterized in that the said shutter element provides for yielding zones in its spherical portions disposed downstream of the said seats with respect to a pressure zone.
Furthermore the valve provides that the said yielding zones are provided by notches produced on surface zones of the said shutter element in correspondence with the said spherical portions.
Preferably the said notches are substantially produced according to a direction parallel to an axis of rotation of the said shutter element.